Air/fuel control systems are known in which fuel delivered to the engine is adjusted by a feedback variable derived from an exhaust gas oxygen sensor to maintain a desired air/fuel ratio. An approach to concurrently control air/fuel ratio while monitoring the converter for degraded operation is disclosed in U.S. Pat. No. 5,268,086. Air/fuel operation proceeds for a time with the feedback variable derived from a catalytic exhaust gas oxygen sensor and then proceeds for another time with the feedback variable derived from a non-catalytic exhaust gas oxygen sensor. The output of the two sensors are compared to determine converter fault.
The inventors herein have recognized numerous problems with the above approaches. One problem is that only a go or no go indication is provided rather than an indication of actual converter efficiency. It is preferable to know the actual converter efficiency so that measures can be taken to improve converter efficiency. Another problem with those prior approaches which substituted a non-catalytic exhaust gas oxygen sensor in the feedback loop during engine air/fuel control, is that such sensor will shift engine air/fuel ratio lean, thereby increasing emissions.